This invention relates generally to the manufacture of carbonaceous materials such as carbon and graphite, and more specifically relates to a process for substantially reducing the porosity and thereby increasing the density and strength of bodies comprising such carbonaceous materials.
The manufacture of artificial graphite, including, for example, the manufacture of graphitic electrodes for use in the chemical and metallurgical industries, is based upon the use of porous petroleum coke as raw material. Since coke porosity has a direct bearing upon the properties of the graphitic end product, it is of economic significance to reduce the porosity of the carbonaceous artifacts after initial baking thereof by impregnating same with a tarry hydrocarbon. Thus the impregnation of carbonaceous bodies, such as the aforementioned electrodes, with such tarry hydrocarbons as coal tar, coal tar pitch, or petroleum pitch, is a well-known process, and is presently being used extensively in the carbon and graphite industry for the purpose of reducing the porosity and permeability of carbon and graphite.
Considering by way of example the large baked electrodes which are customarily utilized in arc furnaces and similar metallurgical environments, the conventional pitch impregnation procedure involves two principal processing steps: i.e. a vacuum-pressure impregnation of the baked electrode with pitch, and a subsequent sagger rebaking of the pitch-impregnated electrode. Thus in a typical procedure baked electrodes to be impregnated are initially placed in a suitable open-mesh carrier within a preheating oven and heated above the melting point of the pitch impregnant. The carrier and preheated material is then transferred into an autoclave which is preheated to temperatures substantially above the melting point of the impregnating pitch. The chamber is sealed and evacuated to remove air from the accessible pores. While maintaining the vacuum, pitch is drawn in until it completely covers the materials to be impregnated. An over-pressure is then applied to drive the impregnant into the pores of the electrode stock. The electrodes are subsequently removed from the autoclave and allowed to cool, and subsequently the pitch impregnated electrodes are heat treated (i.e. rebaked) to a temperature (650.degree. C to 850.degree. C) which will carbonize the pitch impregnant in the pores of the electrodes. After cooling the impregnated and rebaked electrodes are ready for graphitization, as desired.
It is well known that the coke yield obtained from rebaking of pitch-impregnated electrodes, is consistently below the actual coking value of the impregnant pitch by approximately 30%. This low percentage coke yield is considered an inherent shortcoming of the process, and is caused by exudation of the pitch from the impregnated electrode during the heat treating which precedes carbonization of the impregnant.
At least one proposal has been forthcoming, directed at increasing pitch retention during rebake of the electrodes, to thereby result in increased density in the rebaked material. Thus, in U.S. Pat. No. 2,911,319 shaped and baked carbonaceous products are impregnated and baked until coking of the impregnating agent is effected, by keeping the shaped bodies immersed in pitch during the coking procedure.
By and large, however, the prior art techniques, including that discussed above, have been found relatively unsatisfactory.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a method for increasing density and strength in baked carbonaceous bodies such as baked carbon and graphite electrodes or the like.
It is a further object of the present invention, to provide a method which minimizes the losses normally suffered by impregnated carbonaceous bodies, where such bodies following impregnation with pitch compositions are subjected to a rebaking schedule with consequent loss of portions of the impregnant due to exudation of such material from the body during early portions of the rebaking cycle.
It is a still further object of the invention to provide a method which enables the pitch or similar tarry hydrocarbon compositions impregnated into a carbonaceous body to be more effectively retained within the body during the rebaking which effects impregnant carbonization; which method thereby provides increased density and strength in the said bodies; and which method, further, promotes greater uniformity of density and strength throughout the treated body than would be achieved absent use of the invention.